In a surface-discharge AC panel, which is a typical plasma display panel (hereafter “panel”), numerous discharge cells are formed between a front panel and rear panel facing each other. In the front panel, display electrodes, respectively consisting of a pair of scan electrode and sustain electrode, are disposed parallel to each other on a front glass substrate, and they are covered with a dielectric layer and a protective layer. In the rear panel, data electrodes are disposed parallel to each other on a rear glass substrate, and they are covered with a dielectric layer. Barrier ribs are formed parallel to data electrodes, and a phosphor layer is formed on the surface of the dielectric layer and a side face of the barrier ribs. Then, the front panel and the rear panel are disposed facing each other in a way such that the display electrodes and the data electrodes are orthogonal to each other, and sealed. Discharge gas is filled in a discharge space inside. Discharge cells are created at areas where display electrodes and data electrodes face each other. In the panel as configured above, ultraviolet rays are generated by discharging gas inside each discharge cell. These ultraviolet rays excite and make RGB phosphors emit light so as to achieve color display.
For driving the panel, a subfield method is generally used. More specifically, a one-field period is divided into multiple subfields, and grayscale images are displayed by the combination of subfields to emit light. In the subfield method, a new drive method is disclosed in Unexamined Japanese Patent Publication No. 2000-242224. This method is to suppress an increase in luminance of black level by extremely reducing luminescence not related to the grayscale display so as to improve the contrast.
The subfield method is briefly described below. Each subfield includes an initialization period, write period, and sustain period. In the initialization period, either all-cell initialization or selective initialization takes place. The all-cell initialization is to generate initialization discharge in all discharge cells that display images, and selective initialization is to generate initialization discharge selectively only in discharge cells where sustain discharge is generated in a preceding subfield.
In the all-cell initialization period, initialization discharge is generated at once in all discharge cells for erasing a past record of wall charge in each discharge cell and forming a wall charge needed for a subsequent write operation. The initialization discharge also acts to generate priming (spark for discharge=priming particle) for reducing discharge delay so as to stabilize write discharge. In the selective initialization period, a wall charge required for the write operation is formed in a discharge cell where sustain discharge has been generated in a preceding subfield. In a subsequent write period, a scan pulse is applied sequentially to the scan electrodes, and a write pulse corresponding to a picture signal for display is applied to the data electrodes so that selective write discharge is induced between the scan electrodes and data electrodes for selectively forming the wall charge. In the sustain period, a sustain pulse is applied between the scan electrodes and sustain electrodes for a predetermined number of times corresponding to each level of brightness weight so that discharge cells where the wall charge is formed by write discharge are selectively discharged for luminescence. By reducing the number of subfields undergoing all-cell initialization, luminescence not related to grayscale expression can be reduced for suppressing the increase in luminance of the black level.
Here, it is important to ensure reliable selective write discharge in the write period for displaying images correctly. However, there are many factors that significantly delay a write discharge: No high voltage is applicable to the write pulse due to restrictions of the circuit configuration, the phosphor layer formed on the data electrodes hinders discharge, and so on. Accordingly, priming for reliably generating a write discharge is extremely important.
In the plasma display device, the wall charge that is stored in an off-cell weakens due to write discharge and/or luminescence sustain discharge in an on-cell because discharge interference occurs between these discharge cells if the adjacent cells are an on-cell and off-cell in a specific subfield. As a result, in the discharge cell with weakened wall charge, the sum of the pulse voltage applied to each electrode in the write period of a subsequent subfield and wall charge falls to below the discharge start voltage. This hinders a correct write operation, and the predetermined discharge cell becomes a dark spot, notably deteriorating picture quality.
The degree of weakening of wall charge by discharge interference between adjacent cells is proportional to the number of luminescence sustain operations. Accordingly, the wall charge is more apparently weakened in subfields with large weight. This discharge cell with weakened wall charge as described above cannot recover its correct write and luminescence sustain operations until one entire field is completed, and this causes deterioration of picture quality.
The present invention counteracts this disadvantage, and offers a plasma display device that ensures correct write operation and high contrast.